The same properties that allow T cells to mount an effective immune response can be turned against host tissue, provoking an autoimmune response. Intriguingly, T cell auto-reactivity is often the result of deregulated signaling pathways within T cells. Thus, understanding the etiology of different autoimmune diseases will require a thorough and integrated understanding of the mechanisms that control T cell activation. The long-term objective of my research is to understand the regulation of T cell activation. Currently, my laboratory is studying how two related proteins, Sts-1 and Sts-2, act in concert to negatively regulate T cell signaling pathways. The Sts proteins have a unique modular structure, with an N- terminal ubiquitin-interacting domain (UBA), a central protein-protein interaction domain (SH3), and a C- terminal region with homology to the enzyme phosphoglycerate mutase (PGM). The realization that the Sts proteins have a role in regulating TCR signaling pathways emerged from an analysis of mice lacking Sts-1 and -2. T cells from Sts-1/2-/- mice dramatically hyper-proliferate in response to TCR stimulation. This hyper- proliferative phenotype is accompanied by increased activation of signaling pathways downstream of the TCR, elevated levels of cytokine production, and increased susceptibility of Sts-1/2-/- mice to autoimmunity in a mouse model of multiple sclerosis. In our ongoing effort to characterize the functions of the Sts proteins, we recently discovered a novel protein tyrosine phosphatase activity associated with the Sts PGM domain. Intriguingly, the Sts phosphatase domain shows no similarity to classical PTPs in primary amino acid sequence and important catalytic features. Coupled with the fact that no known PTP contains a UBA or SH3 domain, our results suggest that the Sts proteins operate in an intracellular signaling niche within T cells that is separate and distinct from classical PTPs. The experiments outlined in this proposal are designed to address the underlying function(s) and mechanism(s) of action of the Sts-1 and Sts-2. Our Specific Aims are: 1. Determine how the Sts proteins regulate T cell activation thresholds via regulation of Zap-70. 2. Determine how the three Sts modular domains (UBA, SH3, PGM) cooperate to negatively regulate TCR signaling pathways. We will use a combination of biochemical, biophysical, and cell biological approaches to accomplish our goals. Completion of the studies described herein will help us build a model of how the Sts proteins cooperate to negatively regulate TCR signaling pathways.